package org.terst.nav.nmea

import org.terst.nav.gps.GpsPosition
import java.util.Calendar
import java.util.TimeZone

class NmeaParser {

    /**
     * Parses an NMEA RMC sentence and returns a [GpsPosition], or null if the
     * sentence is void (status=V), malformed, or cannot be parsed.
     *
     * Supported talker IDs: GP, GN, and any other standard prefix.
     * SOG and COG default to 0.0 when the fields are absent.
     */
    fun parseRmc(sentence: String): GpsPosition? {
        if (sentence.isBlank()) return null

        // Strip optional checksum (*XX suffix)
        val body = if ('*' in sentence) sentence.substringBefore('*') else sentence

        val fields = body.split(',')
        if (fields.size < 10) return null

        // Sentence ID must end with "RMC"
        if (!fields[0].endsWith("RMC")) return null

        // Status must be Active; Void means no valid fix
        if (fields[2] != "A") return null

        val latStr = fields[3]
        val latDir = fields[4]
        val lonStr = fields[5]
        val lonDir = fields[6]

        if (latStr.isEmpty() || latDir.isEmpty() || lonStr.isEmpty() || lonDir.isEmpty()) return null

        val latitude = parseNmeaDegrees(latStr) * if (latDir == "S") -1.0 else 1.0
        val longitude = parseNmeaDegrees(lonStr) * if (lonDir == "W") -1.0 else 1.0

        val sog = fields[7].toDoubleOrNull() ?: 0.0
        val cog = fields[8].toDoubleOrNull() ?: 0.0

        val timestampMs = parseTimestamp(timeStr = fields[1], dateStr = fields[9])

        return GpsPosition(latitude, longitude, sog, cog, timestampMs)
    }

    /**
     * Converts NMEA degree-minutes format (DDDMM.MMMM) to decimal degrees.
     * Works for both latitude (DDMM.MM) and longitude (DDDMM.MM) formats.
     */
    private fun parseNmeaDegrees(value: String): Double {
        val raw = value.toDoubleOrNull() ?: return 0.0
        val degrees = (raw / 100.0).toInt()
        val minutes = raw - degrees * 100.0
        return degrees + minutes / 60.0
    }

    /**
     * Combines NMEA time (HHMMSS.ss) and date (DDMMYY) into a Unix epoch milliseconds value.
     * Returns 0 on any parse failure.
     */
    private fun parseTimestamp(timeStr: String, dateStr: String): Long {
        return try {
            val cal = Calendar.getInstance(TimeZone.getTimeZone("UTC"))
            cal.isLenient = false

            if (dateStr.length >= 6) {
                val day   = dateStr.substring(0, 2).toInt()
                val month = dateStr.substring(2, 4).toInt() - 1  // Calendar is 0-based
                val yy    = dateStr.substring(4, 6).toInt()
                val year  = if (yy < 70) 2000 + yy else 1900 + yy
                cal.set(Calendar.YEAR, year)
                cal.set(Calendar.MONTH, month)
                cal.set(Calendar.DAY_OF_MONTH, day)
            }

            if (timeStr.length >= 6) {
                val hours   = timeStr.substring(0, 2).toInt()
                val minutes = timeStr.substring(2, 4).toInt()
                val seconds = timeStr.substring(4, 6).toInt()
                val millis  = if (timeStr.length > 7) {
                    (timeStr.substring(7).toDoubleOrNull()?.times(1000.0))?.toInt() ?: 0
                } else 0
                cal.set(Calendar.HOUR_OF_DAY, hours)
                cal.set(Calendar.MINUTE, minutes)
                cal.set(Calendar.SECOND, seconds)
                cal.set(Calendar.MILLISECOND, millis)
            }

            cal.timeInMillis
        } catch (e: Exception) {
            0L
        }
    }
}